The Rotunda was built at the northeastern part of the city, close to the east city walls. Its location was previously an uninhabited, sparsely built area, which was added to the city during the 3rd century fortification. The building is located on the axis of the road that was leading through the Arch of Galerius to the royal palace to the south. It was actually connected to this Arch through a monumental colonnaded way and consequently to the palace.
It was travelers in the 18th and 19th centuries that gave the monument its common name, i.e. Rotunda, due to its circular plan. The monument has a rich history of more than 15 centuries, during which its function and shape changed. These phases, that are the theme of an ongoing scientific debate, will be briefly presented here.
The first Roman phase of the monument began when it was initially built in the Late Roman period, in the late 3rd or early 4th century AD.

The Rotunda’s initial arrangement was like that of the Pantheon in Rome. It had a circular plan, with a diameter of 24,5 meters and 6,3 meters thick walls. It was constructed with bands of brick and rubble made from local greenish-white stones, with mortar binding. Eight niches were opened in the walls approximately 5,0 meters in depth, while colonnaded screens were dividing the niches from the central space. The southern niche opened into a propylon. There was a row of eight large windows above these big ground-floor niches, while nine smaller lunette windows also pierced the upper part of the structure. The building was roofed with a dome and was located inside a precinct.
The dome today consists of two distinctly different zones, the lower one with a regular hemispheric curvature, while the upper one with a slightly steeper curvature. This difference is explained as different construction periods of the parts of dome. Indeed, researchers state that the dome may have originally remained unfinished or it may have needed repair after an earthquake.
As for the founder of the building, the most widespread scientific opinion states that the Rotunda formed part of a large palace complex of the tetrarchic emperor Galerius and was built between 298/299 and 311. Galerius set his residence in the city and was a known persecutor of Christians. The exact function of the building during the time of Galerius is unknown. Researchers claim that it would have been used as a pagan temple, probably dedicated to Jupiter, while others state that it was probably intended as the emperor’s mausoleum, but it was never used as such. Contrary to the above, another recent scientific research states that the building along with the famous mosaics of the dome is attributed to the emperor Constantine I (306-337 AD).

The second Christian phase of the monument is not known when exactly it began. Researchers place the time of the conversion of the Rotunda into a Christian church between the late 4th and 6th century AD. This transformation included the following changes that may have been realized in various phases: a) the completion or reconstruction of the dome, b) the erection of mosaics in the dome, c) the marble revetment of the interior, d) the opening up of the original eight niches, e) the transformation of the eastern niche into a sanctuary, f) the addition of a circular ambulatory, a perimeter corridor, which had access to the main nave through the opened up niches and surrounded the original building and g) the addition of three annexes, one by the west entrance and two by the south entrance, serving as a baptistery and a circular burial chapel. Over the next centuries the Rotunda was renovated several times for Christian use, especially after destructions caused by earthquakes. Thus, the addition of a new entrance to the south with a narthex dates to the 7th century, while the Ascension painting of the sanctuary apse to the 9th century. None of the Christian additions of Byzantine era, apart from the sanctuary and the mosaics, remains till today due to destructions caused by earthquakes.
The Christian church is thought to have served as a palace church or, less convincingly, as a martyrium. Early Ottoman documents and the 16th century travelers’ accounts identify the Rotunda as a church dedicated to Asomatoi (body-less creatures, angels) or Archangels. The church also served as the Cathedral of the city from 1523/4 to 1590/1.
The mosaic decoration of the Rotunda is its most famous feature and of a quality barely equaled. The mosaics are unified, produced by the same artisans and contain 36 million tesserae or so. They are located in the dome, in the lunette windows and in the barrel vaults connecting the ambulatory to the center of the building. The mosaics in the windows and the barrel vaults have an impressive variety of plant and geometric motifs, birds, baskets of fruits, flowers, intersecting circles and squares.
The iconographic program of the dome originally had three zones, of which the lowest is best preserved. The interpretation of the mosaics and their dating is part of an ongoing discussion. Most researchers see in what remains in the top zone the figure of Christ, probably striding forward. Four enormous angels hold this figure of Christ, while in this zone, a phoenix and a rayed cross also appeared. What survives from the middle zone is a green ground with some fragments of sandaled feet and the lower portion of himatia. Researchers claim to recognize twenty-four to thirty-six apostles, prophets, saints, Elders or angels of approximately 3 meters in height. In the lowest zone seven panels remain from the original eight ones. These panels contain two or three male figures each, standing in orans or prayer position, in front of a two-storey golden architectural façade. These figures, sixteen in total, 2,30 to 2,40 meters tall, are accompanied by inscriptions, which note name, occupation and a month. Most agree that the figures are martyrs, saints or founders, donors. When the eastern part of the dome collapsed, the Italian painter S. Rossi replaced the destroyed mosaics with a wall painting in 1889.

On the whole, the original iconographic program of the Rotunda mosaics is though to derive directly from the «classical» Greco-Roman artistic tradition and to be related to the parousia or second coming of Christ. Contrary to the above, recent scientific research states that the famous mosaics of the Rotunda’s dome are not related to its later conversion into a Christian church and are earlier than that. This opinion is based on the fact that the style and themes of the mosaic decoration of the dome was not extended to those portions of the building that underwent Christian structural interventions. Moreover, the features of the dome’s mosaics are related more to imperial cult than to Christian worship. Thus, the whole building along with its mosaic decoration is lately attributed to the emperor Constantine I, who built it as his third burial place, since the Rotunda has all the features of early Christian mausolea. It is supposed that Constantine, though, as he was expelled from his mausoleum in Constantinople, he was expelled from the Rotunda, too, when it was converted into a Christian church.
In 1590-1 the Ottoman Sinan Paşa and the sheikh of the nearby dervish community, Hortaci Süleyman Efendi, converted the building into a mosque named after the latter. The alterations in the building that took place during its ottoman period and that are still visible comprise: a) the addition of a minaret to the south-west corner of the building, which is the only existing minaret in the city till today, b) the addition of a fountain to the west of the building that was needed for the purification ceremony of the Muslims, c) the portico of the western and southern entrance, d) the inscription on the lintel over the west entrance and e) the burial of two people within the precinct of the building, the tombs of whom can be found in an enclosure just outside the church, Hortaci Süleyman Efendi and a certain Yusuf Bey.
After the liberation of Thessaloniki in 1912 and its annexation to the Greek state, the building was turned into a Christian church again until 1914. The name associated with the Rotunda since then, that of St. George, is the result of its proximity to another chapel of that name, where the church vessels were stored after the Rotunda was converted into a mosque. After 1914 extensive archaeological research began along with an ongoing debate about the building’s appropriate use: the clerics state it should be used as a serving church, while the archaeologists proclaim that because of its different original use and its very important internal decoration, the building should be treated as a monument. In 1917 the building was turned into a Macedonian Museum, later it housed an exhibition of Christian statuary, while today it is a monument protected by the Greek Ministry of Culture and simultaneously it serves as a church 3-4 times a year. Restoration plans were put in effect early in the 1950s and especially after the earthquake of 1978. As a result of the 1953 cleaning works a charcoal preliminary drawing of the figures of Christ and the four flying angels has been revealed.
In the yard around the Rotunda there are marble fragments from the Byzantine church, Jewish tombstones and other artifacts. Moreover, one can see the base of the church’s ambo near the south entrance, where it was found during excavations in 1918. Two more surviving sections of the exquisite marble ambo of the Rotunda are now in the Archaeological Museum of Istanbul. The ambo was shaped like an open fan and decorated with scenes from the Adoration of the Magi.
Today, the Rotunda is one of the oldest buildings in the city and a unique monument in Greece in general because of its round shape.

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The monument was selected to be 3D digitised within the framework of the European Commission funded 3D-ICONS project by the Athena RC Xanthi's Division. It's exterior was captured in 3D using the Structure-From-Motion (SFM)
and Dense Multi-View 3D Reconstruction (DMVR) methods. Aerial and terrestrial photoshooting sessions were performed during the winter of 2014 always during cloudy days. A total of four digitisation
sessions were performed where each resulted a number of image sequences presenting the monument from different points of views. The terrestrial photoshooting sessions were implemented using camera tripods
and custom monopods (3m-9m adaptive). We attempted to keep the distance between two terrestrial photoshooting points at 60cm. All aerial photoshooting was performed using a custom hexacopter equipped with gimbal remote controlled two axis tilt-roll camera base, first-person view (FPV) and GPS.
A pair of Samsung NX1000 20MP compact DSLR cameras with fixed 16mm and 20-50mm lenses were used. Agisoft PhotoScan (ver. 1.1) has been used for the production of monument's 3D digital replica. Although, flight paths were designed they
have been performed manually due to bad GPS reception (high buildings around the monument). Furthermore, the calibration of the cameras was performed by the software itself. A total of 10,000 photos were used to generated the 3D model. The processing of the image sequences was performed on two computer systems (CPU IntelCoreI74820K 3.7Ghz, 64GB RAM, Radeon R9 290X,
Microsoft Windows 7 64-bit).
Additionally, empirical measurements between strong (in terms of visibility) feature points on the surface of the monument were performed and used to scale the 3D model.

Ćurčić S., Christianization of Τhessalonike: The making of Christian ‘Urban Iconography’, L. Nasrallah, Ch. Bakirtzis and St. Friesen (eds), From Roman to Early Christian Thessalonike: Studies in Religion and Archaeology, Harvard University Press,2010, 215-218.

Copyright 2014 Athena Research Centre. All rights reserved. The 3D ICONS project is funded by the European Commissions ICT Policy Support Programme.
For technical enquiries and additional information related to the 3D-ICONS project or the 3D data displayed in this page please contact Dr. Anestis Koutsoudis at akoutsou at ceti dot gr.